The present invention relates to video compositing systems, and more particularly to video compositing systems using a software controlled linear keyer for a character generator which provides anti-aliased characters, translucent letters and fades and dissolves without requiring an external switcher or keyer.
In a video production environment one video source must often be matted or composited over a second video source. A video keyer is generally used to produce the composite image. Character generators and other graphic devices often supply a key signal in addition to their normal color outputs. The key signal is used to cut a hole in the background video. When viewed on a monitor the key signal looks like a high contrast, black and white version of the graphic image. The key signal is white for the portions of the screen where the graphic image would obscure the background video, and black where the background video is to be seen. The key signal and color outputs are input to a video keyer which uses the key signal to switch between the foreground graphic image, and a background video source to produce a single video signal output, the matted video.
Key signals produced by traditional graphic devices have been hard edged, i.e., the foreground video source is either on or off and never in-between as illustrated in FIG. 1A. Unfortunately the sharp transition between foreground and background video can cause "chroma-creep" and other distracting artifacts in the composite image. Soft keyers were developed to soften the edge between foreground and background video. Two controls, clip and gain, transform the sharp-edged hard key signal into a smooth, ramped key signal. The clip control chooses a threshold value above which the key signal is considered on and below which the key signal is considered off. The gain control specifies the slope of the output key signal. The ramped key signal, illustrated in FIG. 1B, is then fed to a linear keyer to produce a weighted average between the foreground and background video based on the luminance level of the incoming key signal.
The jagged edges produced by traditional character generators are caused by the hard transition between background video and foreground characters. These transitions take place at pixel boundaries, typically 25 to 50 nanoseconds apart. A properly anti-aliased character has its edges blended with the background. The amount of blending varies from pixel to pixel and depends upon the shape of the character. The anti-aliasing computations required for a self-contained graphic display, i.e., no background video, use well know digital signal processing techniques. Soft keyers reduce the "jaggies" by blurring the edges of the characters. Since a soft keyer has no knowledge about the true shape of a character, aliasing artifacts still exist. Correct anti-aliasing requires that the near vertical edges of a character exhibit a fast transition, one or two pixels from off to on, while near horizontal edges exhibit a slow transition, as long as twenty pixels. Serious problems are often encountered when an anti-aliased display is matted over background video. A hard keyer fringes the foreground graphic with its original background color. Although a soft keyer mutes this problem somewhat, it still allows portions of the original background color to bleed through.
What is desired is a video compositing system which eliminates anti-aliasing artifacts while providing the capability of performing new compositing effects independent of external video keyers.